There is an incomplete 'chessboard' in the fireplace-elevator, which takes you down to the two books. Number the fields on the incomplete 'chessboard', starting at the bottom far left corner, going left to right: 1 to 8. Then for the next row: 9 to 16, and continue the pattern with 48 in the top far right corner. Try pressing: 9-34-6-16-30-19-20.